A kid tells his teacher "You only teach useless crap!"

A kid tells his teacher "You only teach useless crap!"
She replies "Don't be so hard on yourself."

This joke is an insult and probably also an example of someone intentionally misunderstanding someone else. The insult is the teacher assuming that "you only teach useless crap" meant "the only people that you teach are useless and crap" instead of the more likely parsing of the sentence "the only things that you teach to people are things that are useless and crap".

The reason why we know that the teacher made that assumption is because their statement "don't be so hard on yourself" is the teacher telling the student to not insult themself.

The second parsing of the sentence is significantly more likely, so the teacher might have intentionally misunderstood it, and misunderstandings (intentionally or not) can be funny as well.

I chuckled when I read this.